Implement double minimum potential tests

These tests implement shooting solver (dftatom) results for a double minimum
potential, together with finite difference (FD) results. Cubic spline, Lagrange
and Hermite interpolation has been implemented.

This test depends on Lapack, so it is not compiled by default unless enabled
with:

cmake -DWITH_LAPACK=yes .
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Update README with the double_min test
Merge pull request #4 from certik/double_min_potential

Implement double minimum potential tests